Data communication between separate locations usually utilizes a common carrier such as a telephone company that provides a chain of facilities serially linked by telephone lines, radio, or fiber optic lines between the two locations. A typical situation is shown in FIG. 1 wherein a sending customer using a modem sends digital data to a receiving customer along a telephone cable to a central telephone office where the digital data are combined, typically, with twenty-three other inputs in a channel bank which performs a time division multiplexing operation on the input data. Conventionally, the channel bank accepts data from each of twenty-four customers in eight bit blocks. These are assembled into a frame (192 bits) with a single frame bit which contains the parity bit for the frame. Each frame of 193 bits (a DS-1 frame) is sampled 8000 times per second producing a digital signal of 1.54 MBPS.
To permit communication with low signal-to-noise ratios, DS-1 signals transmitted between a central office and a toll office are transmitted as a bipolar bit stream in which the polarity of succeeding pulses is reversed so that the ONE's of the stream alternate as positive or negative levels about the ZERO level. By reason of this arrangement, a DS-1 signal received at the toll office may be monitored by a bipolar violation monitor that produces a logic level output in response to each detection of a bipolar violation in the signal. That is to say, each time successive pulses in the digital signal are of the same polarity, the monitor produces a logic level signal because it is assumed that this detected situation arises due to external events such as noise operating on the signal. The output of the bipolar violation monitor is applied to an alarm control unit at the toll office and serves to close relay contacts that cause illumination of a lamp indicating the existence of this type of problem.
At the toll office, the DS-1 signal containing the data from the sending customer is combined with twenty-seven other similar signals in a digital MUX which performs a time division multiplexing to produce a DS-3 signal. The digital MUX includes a parity generator that analyzes the data processed by the MUX and produces a parity bit for each DS-3 frame thereby creating a 45 MBPS data stream. Two DS-3 signal channels and an overhead channel carrying interfacility communications and control information not available to the public are inputted to what is termed a second generation digital radio modulator operating at 6.6 GHz. Thus, this carrier is modulated with the three input signals and is beamed to a repeater facility typically 10-20 miles away.
At the repeater facility, the radio signal is demodulated to recover the two DS-3 signals and the signals on the overhead channel, the parity bits being stripped from the DS-3 signal and analyzed by a parity checker circuit. Conventionally, the parity checker circuit produces a logic level signal in response to detection of a parity error. The logic level signal drives an alarm control unit which includes a number of relays that are closed in response to certain conditions. Contact closure results when a parity error occurs and this is evidenced by illumination of the lamp on the alarm control unit. The unit has counters that provide information on the bit-error-ratio (BER) or the number of errors per second, and indicates a major alarm when the BER exceeds a 10.sup.-3 threshold, or a minor alarm when the BER exceeds a 10.sup.-6 threshold. Contact closure also occurs upon loss of a signal. In all cases, the parity checker produces a logic level signal which is analyzed by the alarm control unit.
After parity checking occurs, the parity bits are stripped from the signals which are processed to rebuild them; and new parity bits are substituted. The reconstituted signals in the three channels are then used to modulate another 6.6 GHz carrier, and the resultant RF signal is broadcast to a downstream repeater facility.
Eventually, the RF signal is demodulated at a remote toll office and the recovered DS-3 signals demultiplexed at a remote central office where they are broken down into their constituent DS-1 signals, one of which contains the sending customer's data. The DS-1 signal is again demultiplexed and the digital signal from the sending customer is sent to the receiving customer via a telephone cable.
The above description represents a typical way in which digital data are transmitted between customers using technology developed and in place for voice communications. As is well know, the human ear and brain cooperate to render intelligible voice information transmitted using this equipment even though a considerable amount of error is introduced into the digital data during transmission. This is the case because of the large degree of redundancy in the information signal for voice communication.
To provide a measure of the performance of this type of equipment when it is used for voice communication, a bit-error-rate surveillance strategy evolved. This strategy is based on manually analyzing the rate of occurrence of parity bit errors in DS-3 signals using an "eye" monitor that displays a qualitative indication of the level of errors in the signal by the flatness of an elliptical pattern on an oscilloscope screen, the pattern being derived by synchronizing the sweep of the oscilloscope with the frame rate of the DS-3 signal, and displaying the amplitude of the signal. This approach was adequate for facility maintenance purposes when voice communication was the dominant portion of digital traffic. Under this condition, the alarm provisions of digital transmission systems focused on "outage" problems that occurred when the BER exceeded 10.sup.-3 or 10.sup.-6 thresholds.
With the advent of digital services, the strategy that worked well for voice communication became unacceptable because digital equipment is more sensitive to transmission errors. Bit-error-rates generally accepted as being satisfactory for voice communication purposes are not necessarily acceptable so far as digital data transmissions are concerned. As a result, errors in the transmission of digital data to be used by digital equipment are usually neither detected nor corrected until after customer complaints are received. As a consequence, the telephone industry has shifted to an error-free-seconds alarm surveillance strategy for digital services
In this strategy, the idea is to provide performance guarantees to customers in terms of percent error-free-seconds. For example, a customer may be guaranteed by the carrier that he will receive 99.5% error-free-seconds. Stated otherwise, the carrier is guaranteeing that the transmission system will introduce only one error in 200 seconds of transmission. This type of guarantee works well with fiber optic cable systems, but it is difficult to establish and to monitor in existing span terminal facilities that were designed for voice communications. The problem is that a large number of errored-seconds (ES) may occur in a digital transmission system without causing any alarms because the BER is still lower than the established thresholds.
The conventional method for testing facilities for errored-seconds performance when the facilities span terminals have no built in errored-seconds detection circuitry, is to take the facility out of service and perform an out-of-service testing procedure. The disadvantage of this approach is often that the facility is still working well enough in most cases to provide high grade service to many customers on the facility. Nevertheless, when the facility is taken out of service, customers must be transferred to other facilities prior to the tests, and must be transferred back after testing is complete. These are labor intensive processes that are undesirable in an automated system carrying heavy traffic.
It is therefore an object of the present invention to provide a performance monitoring system that effectively retrofits existing telephone industry equipment to an error-free-seconds alarm surveillance strategy.